1. Field of the Invention
The present invention relates to fabrication processes for fabricating gas discharge display panels, and more particularly, to sequential, in situ fabrication processes for the fabrication of gas discharge display panels.
2. Description of the Prior Art
One of the difficulties encountered in gas panel fabrication processes resides in the fact that, in general, such processes involve a multiplicity of steps which are somewhat cumbersome and require separate handling operations. Such operations, of course, are costly. More importantly, however, is the fact that separate handling operations tend to introduce inpurities and foreign matter into the panel which impurities and foreign matter act to cause the panel operation to be nonoptimal. As is evident, the more the separate handling operations involved, the greater the likelihood that the resultant fabricated display will not exhibit acceptable operating characteristics or long-life, or operate satisfactorily. Accordingly, it is quite apparent that any process which tends to eliminate or combine steps such that the member of separate handling operations is reduced is a significant contribution to the overall objective of producing panels which are reliable and acceptable, i.e., increasing yield.
One of the particular problems encountered in the fabrication of AC gas panel display devices resides in the fact that the layer of refractory material whose secondary electron emission charcteristics are important to device operation and which is deposited upon the dielectric material covering the panel conductive lines is, often, quite active chemically. For example, MgO, which is a desirable secondary electron emitter material, is quite reactive with water and carbon dioxide. In this regard, it is known that the electrical properties of the gas panel are affected by exposure to H.sub.2 O and CO.sub.2. It is evident, that other elements and compounds may likewise act to affect the electrical properties of gas panels. Thus, it is quite important in fabricating gas panels to eliminate or minimize any possibility of exposure of the active region of the panel to impurities such as H.sub.2 O and CO.sub.2, particularly during cooling of the fabricated part.
In the usual mode of fabricating gas panels, the panel, after having been sealed in air, must be baked out under vacuum for an extended period of time in a separate step in order to decompose or deadsorb reacted or adsorbed foreign species upon the panel interior surfaces. Thereafter, the panel is backfilled with the desired gas mixture to approximately 400 torr, generally at room temperature or thereabouts, and then the panel is finally tipped off. Unfortunately, such vacuum bake-out is generally not sufficient to remove the contaminants completely, and the panel must then undergo extensive electrical burn-in, which involves setting up a discharge in order to attain specified and stable operating characteristics. The difficulties with such a process are many. For example, the separate process step of sealing the panels in air tends to cause many of the panels so produced to be unacceptable. The exact reasons for this are not completely understood, but for one it can be conjectured that room air ambient, which includes CO.sub.2 and H.sub.2 O, tends to introduce impurities into the panels so fabricated.
One prior art gas panel fabrication process which endeavored to simplify the above-mentioned process is that described by Wilson in U.S. Pat. No. 3,778,126. Wilson describes an in situ process whereby Wilson attempts to eliminate separate manipulation of the panel for the evacuation and backfilling operations by utilizing a single vacuum oven enclosure for the process. Wilson seals the gas panel within the oven enclosure, filled with a neon/argon gas mixture. Since it is Wilson's prime objective to eliminate the panel tubulation, when the Wilson panel is sealed in the neon/argon environment, the panel has permanently sealed therein the gas mixture to be used for gas discharge operations.
One of the difficulties with Wilson, however, resides in the fact that, although the panel is simply sealed in a neon/argon environment thereby, in theory at least, eliminating bake-out and backfill, the panels so produced still contain contamination even though sealed in a neon/argon environment. This is due to the fact that the single initial pump-down of Wilson is apparently not sufficient to eliminate all impurities. Moreover, during the sealing process, there apparently is a considerable amount of out-gassing of impurities from the various panel materials utilized in its fabrication and from the vacuum chamber. These out-gassed impurities are, then, permanently sealed into the panels produced by the Wilson process.
A further difficulty with the Wilson process resides in the fact that Wilson describes backfilling the evacuated enclosure with neon/argon to approximately 1 atmosphere at room temperature in an apparently closed system. Pressures of approximately 1 atmosphere at room temperature in a closed system produce pressures which are much too high and therefore ineffective for efficient gas discharge display operation at normal sealing, i.e., fusing temperatures (e.g. 500.degree. C), wherein the neon/argon is permanently encapsulated in the panels. As can be seen, since the temperature at which the Wilson panel is permanently encapsulated is fixed by the sealing temperature, room temperature pressure for the panels of Wilson can only be selected by appropriate control of pressure within the enclosure at sealing temperature. In the present invention, the panel sealing, i.e., plate sealing, may be accomplished at one atmosphere gas pressure, for example, independent of the final encapsulated neon/argon pressure for the panel. The encapsulated neon/argon pressure of the panel at room temperature can be controlled by both the tip-off temperature and tip-off pressure.
In addition to the mentioned pressure-temperature difficulties, it has been found that, in practice, the burn-in step purportedly eliminated by the Wilson process is, in fact, necessary to produce acceptable panels fabricated by the Wilson process. Finally, it should be noted that the Wilson purpose of eliminating the tubulation step typically employed in the prior art has one further disadvantage. That disadvantage is that, since the surface/volume ratio of gas panels is large and the absolute volume is small, the tubulation structure itself typically acts to provide a ballast, i.e., additional volume diluting any contamination trapped in the panel. Panel lifetime may be shortened without this ballast.